This invention is directed to a process for forming a seam of two ends of rigid screen, to the resultant closed-loop screen, and to a process of using the resultant closed-loop screen.
It is a known process to treat a fibrous web such as of textile fibers with streams of fluid while the web is supported on a screen surface to convert the fibrous web to a nonwoven textile fabric. In one such process, the screen is medium fine (30 mesh, about 12 wires/cm) to coarse and consists of longitudinal wires interwoven with transverse wires. The fabric formed on the screen conforms to the topography of the surface of the screen. In a preferred process wherein the streams are fine columnar streams of liquid, apertures are formed in the fabric at locations corresponding to the knuckles of the interwoven wires of the screen. The apertures in the fabric are bounded by regions of fiber concentration corresponding to the apertures in the screen and these regions are interconnected by groups of fibers curving over and along the surface of the wires of the screen. In order for the process of converting web to fabric to be continuous, the screen has been formed into a closed loop which is rotated beneath the fluid treatment zone. Heretofore, the closed loop was formed by welding the opposite ends of a length of screen together to form a seam. Unfortunately, the resultant seam had a topography which differed from that of the remainder of the screen and caused a defect in the fabric with each revolution of the screen loop.
Various methods for making seams in Fourdrinier screens in the papermaking art have been disclosed. These methods are not applicable to solving the above-described problem, however, because such methods change the topography of the screen and/or have required flexible, manually interweavable longitudinal transverse wires to make the seam. The screen used in the fluid treatment process described above is rigid in the sense that its wires are too stiff to be manually interwoven into a seam without producing a seam of changed topography.
For example, U.S. Pat. No. 2,893,664 discloses cutting the warp wires of the screen so that all warp (longitudinal) wires protrude from their respective shute (transverse) wire at opposite ends of a length of screen, and soldering these warp wires to opposite sides of a solderable thread. This seam produces a change in the topography of the surface of the screen.
U.S. Pat. Nos. 3,366,355, 3,552,691, and 3,596,858 and German Patent Application No. R 11,073, published June 21, 1956 all disclose forming a seam extending over a large number of shute wires by manual interweaving warp wires with these shute wires. For example, in U.S. Pat. No. 3,366,355, warp wires are joined together at three staggered locations (three-warp pattern) across a seam that is seventeen weft wires wide (FIG. 4). Fourdrinier screens are normally a fine screen, i.e., 55 mesh (about 22 wires/cm), and finer; this patent does disclose, however, a 16 mesh screen (about 6 wires/cm), which is manually interweavable by virtue of the screen being made of plastic filaments.
U.S. Pat. No. 3,552,691 discloses an extension of this approach in which the joints between warp wires are staggered at greater than three locations such as a 4, 5, 6, or 7-warp pattern, across the width of the seam comprising, for example, 64 manually interwoven shute wires (FIG. 9). The warp wires may be butted together either at an intersection with a seam weft (shute) or in the space between seam wefts, and the butted warp ends need not be joined together (column 4, lines 8-12).
In still a later development, U.S. Pat. No. 3,596,858 continues the manual interweaving approach to form a seam, wherein the seam can have a width of 13 shute wires (FIG. 2), and the joint is in a five-warp pattern, with one end of each warp wire in the seam lying beneath a shute wire and the opposing ends of each warp wire in the seam either being spaced apart (FIG. 3) or abutting (FIG. 9). No joining together of abutting warp ends is disclosed. The improvement disclosed in this patent is the provision of more effective drainage of the water through the Fourdrinier screen and reduced marking of the paper made on the screen.
German Patent Application No. R 11,073, published June 21, 1956, discloses the making of a seam in which opposing protruding warp wires from opposite ends of a screen are interwoven with seven auxiliary weft wires as shown in FIG. 1. The opposing ends of the warp wires form a staggered joint across the width of the seam and are overlapped at locations between weft wires and thereafter fused to form enlarged joints.